Inbreeding Depression

Inbreeding depression is the reduction in fitness — survival, fecundity, or competitive ability — that occurs when closely related individuals mate and produce offspring with increased homozygosity. Because deleterious alleles are often recessive, they are masked in heterozygotes but expressed phenotypically in homozygotes. Inbreeding increases homozygosity across the genome, exposing these hidden deleterious variants to selection.

The magnitude of inbreeding depression depends on the genetic load of the population — the number and severity of deleterious alleles it carries — and the history of inbreeding. Populations that have experienced long periods of inbreeding may have purged much of their genetic load through selection, resulting in lower inbreeding depression than genetically outbred populations suddenly forced to inbreed.

Inbreeding depression is a central concern in conservation genetics, where small, fragmented populations face both increased inbreeding and reduced gene flow. The combination is dangerous: inbreeding depresses fitness while lack of gene flow prevents the reintroduction of genetic variation that could rescue the population. Genetic rescue — the deliberate introduction of migrants from other populations — is increasingly used as a management tool to counteract inbreeding depression.

Inbreeding depression is often discussed as if it were a moral failing of small populations — as if they were somehow defective for being inbred. This framing is bizarre. Inbreeding depression is not a disease; it is the statistical consequence of a population's genetic history. The appropriate response is not to lament the population's homozygosity but to restore the gene flow that evolution assumes as a baseline condition. Populations inbreed because we have fragmented their landscapes, not because they have chosen to.